The invention relates to superconducting synchronous electrical machines with a fluid flow ventilation system to cool the stator of the machine.
In the process of producing electricity, power generators create heat that must be dissipated from the generator. Heat occurs in generators due primarily to windage and friction, electric current flow, and time-varying magnetic fields in magnetic structures. Frictional heating occurs as the rotor spins at high speed in the generator. Similarly, heating also occurs as current flows through the rotor and stator coils, and as these coils rotate relative to one another in the magnetic fields of the generator. Losses in the magnetic circuit occur as the magnetic fields change with time in permeable materials, such as for example in the stator core and the rotor poles of a synchronous generator.
Generators are conventionally equipped with cooling systems to transfer heat from the stator and rotor away from the generator. Gas ventilation cooling systems have been used in conventional synchronous machines, such as generators and motors, that do not employ superconducting materials. These gas ventilation systems tightly couple the cooling of the stator and rotor. The ventilation system cools both the rotor and stator by forcing cooling gas through gas passages in the rotor and stator. Conventional ventilation systems have employed forward flow and reverse flows of cooling gases through the stator and rotor.
In a forward flow ventilation scheme (FIG. 1) the cooling gas flows through sections of the rotor and stator in series which creates a tight coupling between rotor and stator cooling systems. In a reverse flow ventilation scheme the cooling gas flows through stator and rotor in parallel, and then mixes in the machine air gap, which also leads to a coupling of the stator and rotor cooling.
Because of the coupling of the cooling of the rotor and stator, conventional ventilation systems have been configured to provide adequate cooling for both the stator and rotor. To provide cooling for the rotor, some compromises may have to be made in a conventional ventilation system with respect to cooling the stator and vice versa. It may be difficult to optimize the cooling of either the stator or rotor with a ventilation system that must provide cooling for both the rotor and stator. Nevertheless, ventilation systems have conventionally provided cooling for both the stator and rotor in large industrial and utility power generators.
In a superconducting synchronous machine the rotor field winding is operated at cryogenic temperatures through a cryorefrigeration system that has its own self-contained cooling circuit. A cold, cryogenic coolant is supplied to the rotor through a transfer coupling. The cryogenic coolant is circulated through a cooling circuit on the rotor where it removes heat from superconducting windings, and returns as heated coolant through the rotor and transfer coupling to a stationary cooling system. The cryogenic cooling system provides effective cooling of the rotor in a superconducting machine.
Contrary to conventional machines where stator and rotor cooling systems are coupled in a single ventilation system, the cooling systems of a cryogenic rotor and the gas-cooled stator may be completely independent. The cryogenic cooling system for a superconducting rotor does not cool the stator. The stator of such a superconducting synchronous machine has a separate stator cooling system.
A stator ventilation system has been developed for a superconducting synchronous machine. The stator of a superconducting synchronous machine is cooled by a reverse ventilation system. Cooling gas, such as air or hydrogen, is drawn from the air gap and pumped through a diffuser, heat exchanger and through the stator core back to the air gap. The air gap is tapered along its axial length to optimize the ventilation flow to the stator. The tapering of the air gap may be achieved by shaping the outer surface of the cylindrical rotor.
In addition, a conventional synchronous machine may be retrofitted with a superconducting rotor. Similarly, a conventional stator and rotor ventilation system may be modified to function as a stator only ventilation system, such as is disclosed here. The rotor is coupled by a cryogenic coolant system. The stator ventilation may have forward or reverse coolant gas flow. The proposed stator cooling systems are independent of the type of superconducting rotor configurations, and can be equally applied to iron-core and air-core superconducting rotors.
In one embodiment, the invention is a synchronous machine comprising: a rotor coupled to a rotor cooling system; a stator around the rotor and separated from the rotor by an annular gap between the rotor and an inner surface of the stator, wherein the annular gap has a variable thickness along a length of the gap, and a stator ventilation system independent of the rotor cooling system, wherein the stator ventilation system forces cooling gases through the annular gap.
In another embodiment, the invention is a superconducting electromagnetic machine comprising: a rotor coupled to a rotor cooling system; a stator around the rotor and separated from the rotor by an annular gap between the rotor and an inner surface of the stator, wherein the annular gap has a variable thickness along a length of the gap, and a stator ventilation system independent of the rotor cooling system, wherein the stator ventilation system forces cooling gases through the annular gap.
In a further embodiment, the invention is a superconducting electromagnetic machine comprising: a solid core rotor cryogenically cooled by a superconducting rotor coil winding; a stator coaxial with the rotor and having stator coils magnetically coupled with the superconducting rotor coil winding, wherein the stator coils are arranged around the rotor, and the stator has cooling passages extending outwardly from an inner periphery of the stator; the inner periphery of the stator being separated from the rotor by an annular rotor gap, wherein the rotor gap has a tapered thickness along a length of the gap; the rotor being cooled by a cryogenic cooling fluid; and a stator ventilation system providing cooling gas to the outer periphery of the stator and the passages of the stator.
In a still further embodiment, the invention is a method for shaping a gap between a rotor and a stator in a synchronous electromagnetic machine, the method comprising the steps of: forming the stator having a cylindrical cavity to receive the rotor, wherein the stator includes cooling ducts open to the cavity; forming a cylindrical rotor surface on the rotor, wherein the rotor surface forms an inner cylindrical surface of the gap, and shaping the rotor surface to taper a thickness of the gap along a length of the gap.